Sort the file list in order, by implementing Comparator.

package com.AndroidListFiles;

import java.io.File;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Comparator;
import java.util.List;
import java.util.Stack;

import android.app.ListActivity;
import android.content.Intent;
import android.net.Uri;
import android.os.Bundle;
import android.os.Environment;
import android.view.View;
import android.webkit.MimeTypeMap;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.Toast;

public class AndroidListFilesActivity extends ListActivity {
 
 private List<String> fileList = new ArrayList<String>();
 
 Stack<File> history;

   @Override
   public void onCreate(Bundle savedInstanceState) {
       super.onCreate(savedInstanceState);
      
       history = new Stack<File>();
      
       File root = new File(Environment
         .getExternalStorageDirectory()
         .getAbsolutePath());
       ListDir(root);

   }
  
   @Override
 protected void onListItemClick(ListView l, View v, int position, long id) {
  // TODO Auto-generated method stub
    File selected = new File(fileList.get(position));
    if(selected.isDirectory()){
     ListDir(selected);
    }else {
     Uri selectedUri = Uri.fromFile(selected);
     String fileExtension
      = MimeTypeMap.getFileExtensionFromUrl(selectedUri.toString());
     String mimeType
      = MimeTypeMap.getSingleton().getMimeTypeFromExtension(fileExtension);
     
     Toast.makeText(AndroidListFilesActivity.this,
       "FileExtension: " + fileExtension + "\n" +
       "MimeType: " + mimeType,
       Toast.LENGTH_LONG).show();
     
     //Start Activity to view the selected file
     Intent intent;
     intent = new Intent(Intent.ACTION_VIEW);
     intent.setDataAndType(selectedUri, mimeType);
     startActivity(intent);

    }
 }

 void ListDir(File f){
  
  history.push(f);
  
    File[] files = f.listFiles();
    
  Arrays.sort(files, filecomparator);
    
    fileList.clear();
    for (File file : files){
     fileList.add(file.getPath());  
    }
    
    ArrayAdapter<String> directoryList
    = new ArrayAdapter<String>(this,
      android.R.layout.simple_list_item_1, fileList);
    setListAdapter(directoryList); 
   }
 
 Comparator<? super File> filecomparator = new Comparator<File>(){
  public int compare(File file1, File file2) {
   return String.valueOf(file1.getName()).compareTo(file2.getName());
  } 
 };

 @Override
 public void onBackPressed() {
  // TODO Auto-generated method stub
  
  history.pop(); //remove current directory
  
  if(history.empty()){
   super.onBackPressed();
  }else{
   File selected = history.pop(); //Get the File on Top of history
   ListDir(selected);
  }
  
 }

}